During the past 20 years the utilization of computer chips has increased dramatically. With this progress has come a subsequent decrease in the size of the chips and an increase in the density of electrical circuits on a given chip. These high-density chips may have power densities as high as 10 W/cm2. With the increase in power density of modern chips has come a concomitant increase in the need to thermally regulate the chips. These chips and other such high-density electrical components generate a tremendous amount of heat which must be dissipated to prevent damage to the chip.
Initially, the heat was dissipated by securing the chip to a heat sink material having high thermal conductivity. Examples of such materials include copper, aluminum, and diamond. One difficulty associated with such solutions is that typically the heat sink material has a much higher thermal expansion coefficient than the silicon chip. For example, the thermal expansion coefficient of silicon is 4 ppm ° C.−1 while the expansion coefficient of aluminum is 24 ppm ° C.−1. Thus, during thermal cycling of the system the aluminum will expand to a much greater extent than the silicon chip. This leads to debonding of the chip from the heat sink.
In an effort to address this difficulty the industry has developed metal matrix composites formed from ceramic preforms that have been infiltrated with molten metal under high temperature and often high pressure to create a metal matrix composite. The difficulty associated with this solution is that the metal matrix composites made in that manner are extremely costly to produce, can only be done with certain ceramic materials, and require inclusion of various compounds such as silicon in the infiltrating metal in order to prevent adverse reactions between the metal and the ceramic. Because the infiltration temperatures are generally in the range of 800° C. or higher reactions between the metal and the ceramic occur that lead to degradation in the thermal conductivity of the final metal matrix composite. The goal of these metal matrix composites is to produce a composite material that maintains the high thermal conductivity of the metallic element while adding the low thermal expansion coefficient of the ceramic to reduce differential expansion and contraction of the heat sink relative to the silicon chip.
In a typical construction of a silicon chip with an attached heat sink the first step is formation of the heat sink laminate. Then the laminate is attached to the silicon chip. The first laminate layer is generally a baseplate formed from a pure metal having a high thermal conductivity such as aluminum or copper that will be placed in the flow of a water stream or an air stream. The second layer is typically a metal matrix layer produced by high temperature infiltration of a molten metal into a ceramic preform and then secured to the baseplate. The third layer is some form of a dielectric material such as alumina, aluminum nitride, or beryllium oxide. The dielectric layer is necessary to provide electrical isolation between the silicon chip and the electrically conductive heat sink. Another metal matrix composite layer may be placed over the dielectric. Finally, another layer formed from copper or other solderable material is attached to the previous layer. Once this heat sink laminate is formed the silicon chip can be soldered to the last layer.
Because of the difficulties associated with current technology for forming metal matrix composites it would be advantageous to produce a metal matrix composite that did not require high temperatures during its production, that could be easily applied to a substrate surface, and that could be easily modified to provide different thermal conductivity and thermal expansion coefficients to the metal matrix composite so that it is optimized for the particular application. In addition, it would be advantageous to develop a system capable of forming metal matrix composites that are impossible to impractical to produce at the present time, such as for example, aluminum diamond metal matrix composites.
A new technique for producing coatings on a wide variety of substrate surfaces by kinetic spray, or cold gas dynamic spray, was recently reported in an article by T. H. Van Steenkiste et al., entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 62–71, Jan. 10, 1999. The article discusses producing continuous layer coatings having low porosity, high adhesion, low oxide content and low thermal stress. The article describes coatings being produced by entraining metal powders in an accelerated air stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate. The particles are accelerated in the high velocity air stream by the drag effect. The air used can be any of a variety of gases including air or helium. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate. It is theorized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and mechanical deformation. Thus, it is believed that the particle velocity must be high enough to exceed the yield stress of the particle to permit it to adhere when it strikes the substrate. It was found that the deposition efficiency of a given particle mixture was increased as the inlet air temperature was increased. Increasing the inlet air temperature decreases its density and thus increases its velocity. The velocity varies approximately as the square root of the inlet air temperature. The actual mechanism of bonding of the particles to the substrate surface is not fully known at this time. It is believed that the particles must exceed a critical velocity prior to their being able to bond to the substrate. The critical velocity is dependent on the material of the particle.
The work reported in the Van Steenkiste et al. article improved upon earlier work by Alkimov et al. as disclosed in U.S. Pat. No. 5,302,414, issued Apr. 12, 1994. Alkimov et al. disclosed producing dense continuous layer coatings with powder particles having a particle size of from 1 to 50 microns using a supersonic spray.
The Van Steenkiste article reported on work conducted by the National Center for Manufacturing Sciences (NCMS) to improve on the earlier Alkimov process and apparatus. Van Steenkiste et al. demonstrated that Alkimov's apparatus and process could be modified to produce kinetic spray coatings using particle sizes of greater than 50 microns and up to about 106 microns.
This modified process and apparatus for producing such larger particle size kinetic spray continuous layer coatings are disclosed in U.S. Pat. Nos. 6,139,913, and 6,283,386. The process and apparatus provide for heating a high pressure air flow up to about 650° C. and combining this with a flow of particles. The heated air and particles are directed through a de Laval-type nozzle to produce a particle exit velocity of between about 300 m/s (meters per second) to about 1000 m/s. The thus accelerated particles are directed toward and impact upon a target substrate with sufficient kinetic energy to impinge the particles to the surface of the substrate. The temperatures and pressures used are sufficiently lower than that necessary to cause particle melting or thermal softening of the selected particle. Therefore, no phase transition occurs in the particles prior to impingement.
The present invention relates to a kinetic spray method of forming metal matrix composites for use in heat sink laminates. The method is capable of quickly producing metal/ceramic composites that were not previously obtainable and applying them to substrates under very low thermal stress. The invention is particularly suitable for thermal management of silicon chips and other high power density electrical components.